Relaying is considered for LTE-Advanced as a tool to improve, for example, the coverage of high data rates for User Equipment (UE), group mobility, temporary network deployment, the cell edge throughput and/or to provide coverage in new cell areas. E-UTRAN supports relaying by having a Relay Node (RN) wirelessly connected to a base station (eNB) (referred to as a Donor eNB (DeNB)). In addition to serving its own ‘donor’ cell, the Donor eNB serves the RN, via a modified version of the E-UTRA radio interface. The modified interface is referred to as the ‘Un’ interface.
Each RN is provided with many aspects of a base station's functionality and is therefore capable of acting as a base station serving its own ‘relay’ cell. From the perspective of the user equipment in the relay cell, therefore, the RN essentially appears to be a conventional LTE base station. In addition to the base station functionality, however, the RN also supports a subset of the UE functionality including, for example, many aspects of the physical layer, layer-2, radio resource control (RRC), and non access stratum (NAS) functionality, to allow it to connect wirelessly to a Donor eNB.
Several mobility management entities (MMEs) are located in the core of the E-UTRA network. Each MME serves a respective subset of the user equipment in the ‘donor’ cell of the Donor eNB, the relay cell, and/or other cells in the wider network. The Donor eNB forwards control messages received from the MME to the RN over the Un interface and, from the perspective of the RN, the Donor eNB appears to be a single MME that serves all the items of user equipment registered with the RN in the relay cell.
Accordingly, the Donor eNB is capable of handling communication both ‘directly’ to and from user equipment camped in its own cell and ‘indirectly’, via the RN, to and from user equipment camped in the relay cell.
The MMEs in the core network can become overloaded and, accordingly, each MME is provided with an overload handling capability. Specifically, when one of the MMEs experiences an overload situation, it randomly selects a subset of the base stations that it communicates with, and sends the selected base station(s) a control message to request initiation of a procedure to alleviate the overload situation. Typically, for example, the requested overload procedure will involve the base station rejecting attempts to establish a connection made by user equipment served by the MME making the request.